1. Field of the Invention
This invention relates to a process and apparatus providing continuous and relatively constant level thermal energy delivery from a periodically active energy source such as solar energy in space applications. The present invention provides the energy supply sector for a power generating sector comprising a power generation means, such as a turbine, a condenser, a pump, and a reheater. The two sectors combine to form a continuously operating working fluid loop to provide continuous delivery of power while the solar energy source is regularly intermittent. The energy supply sector of this invention is intended for use with dynamic solar power systems for orbiting space stations. It acquires solar energy during insolation, transmits it by conductance, buffers the incoming temperature wave, stores some of the acquired energy to be delivered during eclipse, but continuously delivers a supply of working fluid at a relatively constant temperature for an energy utilization sector which may employ a power generation means, such as a turbine.
2. Description of the Prior Art
Utilization of solar energy to power orbiting space stations is desirable since solar energy is available at useful intensities and with cyclic regularity. However, since periods of insolation are regularly followed by periods of eclipse, utilization of this energy in a continuous manner conventionally requires storage of energy during periods of insolation and its release during periods of eclipse. For instance, a solar dynamic power system may comprise a solar collector in which the engine's working fluid is heated during insolation to a level in excess of that needed at the engine inlet. It is then passed through a regenerator in which some of the acquired energy is stored while the temperature of the passing fluid is lowered to that required at the engine's inlet. As the eclipse begins, this stream is shut off, and the cold fluid returning from the engine is passed through the regenerator which heats it to the level required by the engine. At the end of the eclipse the cycle is repeated. Another conventional method for providing continuous thermal energy delivery to a working fluid from a periodically active energy source exposes the working fluid directly to the energy source during insolation, while at the same time charging separate solar energy collectors comprising thermal energy storage material. During eclipse, working fluid is passed in heat exchange relationship with thermal energy storage material, recovering the thermal energy stored during insolation. Both methods require switching of streams and both must rely upon excess regenerator mass to damp out the undesirable temperature fluctuations of the working fluid during the cycle. Solar energy may be transferred to the thermal energy storage material either directly by means of a receiver/concentrator, or by introducing a second fluid requiring a second loop, if the thermal energy storage medium is separate and at some distance from the receiver.
Another way to accomplish the desired energy transfers is to provide two fluids alternately occupying the same space. A first, thermal energy carrying fluid yields its thermal energy to a thermal energy storage matrix, and a second, the engine working fluid, recovers heat stored in the thermal energy storage matrix. Relatively continuous delivery of the second fluid may be approached by operating several regenerative units and substituting the fluid flows at appropriate cyclic intervals. It is difficult, however, to provide working fluid to a power generation means at a constant working temperature with multiple stream switchings.
A continuous supply of relatively constant temperature working media may be approached in conventional stationary applications by using a large mass of thermal energy storage matrix having a large thermal inertia which is only slightly affected by passage of a thermal energy carrying and a thermal energy retrieval or working fluid. This approach is impractical for use in orbiting space stations due to large space and mass requirements.
Phase change thermal energy storage media are known wherein thermal energy storage and release are accomplished utilizing the heat of fusion of the phase change storage material, in addition to utilizing the sensible heat. Such media are capable of storing and releasing more heat energy per unit mass than energy media not undergoing a phase change during their operating cycle. Solar energy absorbed by a suitable phase change thermal energy storage material during insolation may convert some or all of such material from solid to liquid phase. Energy stored may then be released as the latent heat of solidification during solar eclipse as the material is converted from a liquid to a solid state due to temperature decreases. Solid-liquid-solid phase change thermal energy storage media having various latent heats of fusion are well known for use in both residential and industrial applications on Earth.
Solar collectors with a double tube structure are known in which thermal energy storage material changeable from a solid phase to a liquid phase is enclosed in the space between the inner tube and the outer tube with heat transfer fluid introduced through the inner tube, and with solar energy supplied from trough concentrators to the outer tube. Such collectors are exemplified by U.S. Pat. No. 4,469,088 which teaches a round outer tube and a fluted inner tube in contact with the outer tube at the peaks of the flutes for transfer of thermal energy between the inner and outer tube. Such collectors dampen abrupt temperature changes in the heat transfer fluid flowing through the inner tube. Such systems cannot be used for simultaneous energy storage due to long night periods here on Earth, and because of unpredictability of weather changes the system cannot be programmed to deliver a requisite amount of energy to a power generating system. As a result of the partial direct contact of the inner and outer tubes intended to heat the heat transfer fluid without passing through the thermal energy storage material, fluid flowing through the inner tube would be subject to little temperature wave damping in orbital applications and would be prone to exhibit unacceptably large temperature variations at the turbine inlet.
Invariably, conventional art teaches splitting off a portion of the energy stream available during insolation and storing it in a separate thermal energy storage device from which the energy is extracted to be used during eclipse periods.
U.S. Pat. No. 3,903,699 teaches a solar energy power system for space orbital use wherein a low boiling temperature fluid is vaporized in a solar boiler during insolation and used to operate a turbine. A portion of the heat is stored in a phase change material and used to supply heat to generator vapor during eclipse.
U.S. Pat. No. 4,091,622 teaches absorption of solar energy by a collector and its transference by means of suitable piping for storage. When the stored heat is to be later used, as in the nighttime or on overcast days, a switching device uncouples the collector from the storage structure and couples the storage structure to the boiler.
The problem of reducing heat losses from the thermal energy storage material during periods when the solar source is not active is conventionally treated by shielding. U.S. Pat. No. 4,304,218 teaches the use of elongated parabolic reflectors, hinged, and movable in a manner to completely cover the heat receiver tubes filled with a heat collecting medium when a non-active period or cloudiness supervenes.
Other methods are known for maintaining constant temperature conditions for space equipment. U.S. Pat. No. 3,548,930 teaches an isothermal cover to maintain constant temperature conditions for space equipment when the cover is exposed to variable temperature conditions. The isothermal cover is a double-walled shell with containers of phase change material mounted on struts inside, the phase change material absorbing, storing and releasing heat energy, utilizes the heat pipe principle to equalize temperature circumferentially and to conduct heat radially to the inner core. The purpose of the device of this patent is to provide isothermal environment for sensitive instruments or the like by absorbing, holding, and redistributing the incoming thermal energy rather than to serve as a continuous energy source for a designated use, as by a shaft power generator.